The exemplary embodiments described herein relate generally to the useful manipulation of infrared components found in toners as commonly utilized in various printer and electrostatographic print environments. More particularly, the teachings provided herein relate to an improved realization of infrared encoding of data elements or infrared marks across devices.
It is desirable to have a way to provide for the detection of counterfeiting, illegal alteration, and/or copying of a document, most desirably in a manner that will provide document security and which is also applicable for digitally generated documents. It is desirable that such a solution also have minimum impact on system overhead requirements as well as minimal storage requirements in a digital processing and printing environment. Additionally, it is particularly desirable that this solution be obtained without physical modification to the printing device and without the need for costly special materials and media. And importantly it is desirable that the approach can be ported across different devices.
Watermarking is a common way to ensure security in digital documents. Many watermarking approaches exist with different trade-offs in cost, fragility, robustness, etc. Note that here and in the following we are using the common generalized definition of watermark that goes beyond the original paper-based watermark and also includes other materials or digital encoding. One prior art approach is to use special ink rendering where the inks are invisible under standard illumination. These inks normally respond to light outside the visible range and can be made visible either by wavelength conversion or by appropriate sensors. Examples of such extra-spectral techniques include UV (ultraviolet) and IR (infrared). This traditional approach is to render the encoded data with special inks that are not visible under normal light but have strong distinguishing characteristics under the special spectral illumination. Determination of the presence or absence of such encoding may be thereby subsequently performed using an appropriate light source and detector. One example of this approach is found in U.S. Patent Publication No. 2007/0017990 to Katsurabayashi et al., which is herein incorporated by reference in its entirety for its teachings. However, these special inks and materials are often difficult to incorporate into standard electro-photographic or other non-impact printing systems like solid ink printers, either due to cost, availability or physical/chemical properties. This, in turn, discourages their use in variable data printing arrangements, such as for redeemable coupons or other personalized printed media for example.
Another approach taken is where copy control is provided by digital watermarking, as for example U.S. Pat. No. 5,734,752 to Knox, where there is provided a method for generating data encoding in the form of a watermark in a digitally reproducible document which are substantially invisible when viewed. The method generally includes the steps of: (1) producing a first stochastic screen pattern suitable for reproducing a gray image on a document; (2) deriving at least one stochastic screen description that is related to said first pattern; (3) producing a document containing the first stochastic screen; (4) producing a second document containing one or more of the stochastic screens in combination, whereby upon placing the first and second document in superposition relationship to allow viewing of both documents together, correlation between the first stochastic pattern on each document occurs everywhere within the documents where the first screen is used, and correlation does not occur where the area where the derived stochastic screens occur and the image placed therein using the derived stochastic screens becomes visible.
Current methods of providing infrared security elements are based on metameric rendering. However, since metameric matches are strongly dependent on actual machines, only an explicit, same angle halftone method was found portable enough between production level machines. This explicit halftoning can be implemented through a PostScript Pattern Ink construct, as described, for example, in U.S. Patent Publication No. 2008/0302263 to Eschbach et al., which is herein incorporated by reference in its entirety for its teachings. These different explicit patterns have a strong periodic appearance and are collected as user available color palette, e.g., in VIPP (Variable-Data Intelligent PostScript Printware). However, all the colors in the palette are compromises, and it is desirable to produce better IR active colors that have better visual and infrared properties.
Infrared encoding can be obtained by alternating between different metameric renderings of a “color.” A problem in those scenarios was the “color” had to be spatially varying to hide any visual mismatch between the renderings. Additionally, the “color” had to be reasonably stable across devices and thus compromises had to be made, leading to strongly textured “colors,” described as “Pattern Ink” in a PostScript construct.
These patterned colors allow the creator a larger freedom in document design by being able to “hide” any infrared data inside the color field. These patterned inks, however, do not suffice for several design problems where, for example, a corporate letterhead or a photo book is created. For these instances a more homogeneous color is needed. However, homogeneous colors cannot be created using the current IR color approach.
What is needed is a different approach to IR color generation that on the creation side is machine in-dependent but on the rendering side is tuned to the actual machine response. Being able to specify a color (i.e., visual color) that will look the same on different printers is an important attribute of job portability. This color may be rendered with a different machine dependent colorant mixture as stored as a resource on the digital front end (DFE). For the IR case, it is helpful therefore to define a machine resource so that the colors also have minimal IR response, i.e., color pairs that show specific metameric properties.